BRISBANE, Australia—Islamic State-aligned militants’ onslaught in the southern Philippines should be a wake-up call to Asia that battle-hardened jihadists returning from the Middle East are seeking to open new fronts in the region, the commander of U.S. Pacific forces said.

Adm. Harry Harris, in a speech to the Australian Strategic Policy Institute Wednesday, said Islamic State was a “nemesis to humanity” that threatened to spread as it loses ground in Syria and Iraq.